Within the automobile industry the use of electrically heatable exterior mirrors is on the increase. Heated seats as well are more and more widespread. In order to achieve the desired heating in such applications, in the simplest case copper wires are laid in a planar fashion. In recent years the use of what are called PTC elements (PTC for “positive temperature coefficient”) has become established. For instance, for exterior auto mirrors, PTC elements contacted with aluminum, for example, are adhesively bonded. As a result of a defined current strength being applied the PTC element heats up and the heat is transferred via a double-sided pressure-sensitive adhesive tape to the glass surface of the mirror. In this way it is possible to obtain temperatures of 45 to 60° C. on the surface. Besides a very high thermal conductivity, particular requirements are also imposed on the pressure-sensitive adhesive tape in respect of thermal shear strength at elevated temperatures, weathering stability, and adhesive tack at low temperatures.
The existing concept functions well but requires a relatively complicated construction, since the PTC elements must be bonded not only to the glass of the mirror but also to the support plate of the mirror, which in many cases consists of the plastic acrylonitrile/butadiene/styrene (ABS). The bonding of these different materials likewise places particular requirements on the adhesive.
To simplify the operation of manufacturing heatable mirrors, therefore, there is a need for a heatable pressure-sensitive adhesive or adhesive tape which bonds the support plate to the mirror and can also be heated by means of electrical current.